Currently in the back end of semiconductor processing, a solvent bath is used to treat and clean surfaces. In this part of the processing, the semiconductor wafer has metal layers. “Backend” refers to the portion of semiconductor manufacturing after interconnect metal layers are deposited in the processing flow.
A number of references detail systems for use in semiconductor processing baths. Some of these systems use conductivity as a parameter for system function. For example, U.S. Pat. No. 6,392,417 discloses a system and method for determining the end of life of a chemical bath. A chemical species is added at determined time intervals. A parameter of the chemical bath, such as conductivity or pH, is measured. This is compared to a predicted value at a specific time in the bath life cycle. The end of the life of the bath is determined by the difference between the predicted value and the measured value at the time of measuring.
U.S. Pat. No. 6,767,877 discloses a silicon wafer manufacturing process that includes processing a wafer in a process tank. Hot and cold deionized (“DI”) water may be supplied to the process tank at a specified supply rate to be mixed with a supplied chemical. A conductivity sensor determines the conductivity of the mixture before the mixture is added to the process tank.
U.S. Pat. No. 6,551,422 discloses a more generalized device for preparing metal for painters that includes multiple bath stages. Several baths include a conductivity sensor and some also include a pH sensor. This system is described as a system to wash and condition metal prior to painting.
Current practice is to fill a strip bath with the required solvent, allow the bath to be used in a process to clean semiconductor surfaces, and periodically top off the solvent in the bath. For example, a bath may be topped off every three hours of bathlife. The “top-off” solution is used to replace solvent compounds that have volatilized. This replaces both water that has evaporated and solvent components that readily volatize at the bath temperature. Although the top-off solution is primarily water, it is nonetheless very expensive.
The solvent composition changes over the lifetime of the bath. The solvent tends to deplete in the more readily volatized components until such components are replenished. This creates non-uniformity in the process conditions.
One possible solution that has been evaluated is to periodically make a titration unit measurement on the solvent bath throughout the solvent bath life. For example, a titration measurement may be run every hour. The resulting measurement would then be used to determine how much water to add to the solvent bath. The capability of this type of a control scenario is limited by the speed of the sample acquisition and titration equipment (sampling frequency).
It is one goal to provide a system and method in which the volatilized compounds lost during the use of a solvent bath may be readily replaced during the use of the bath. It is a further goal to obtain this using a system that is robust, provides continuous measurements, is able to detect system anomalies, and is automated.